Hair is widely distributed in the skin. Hair is believed to serve a number of biological or physiological purposes. These include thermal protection, as well as protection from abrasion and from sun exposure. Hair also plays a role in sensory reception, most particularly the sense of touch. The appearance of hair also plays a role in visual signaling.
Humans have significantly less hair than other mammals, and its functions are commensurately reduced. However, the visual signaling role of human hair, sometimes referred to as aesthetic appearance, is undiminished.
Hair is part of a complicated anatomical structure that is sometimes referred to as the hair follicle. The hair shaft, which protrudes from the skin, is, for a practical purposes, the only portion of the hair follicle that is visible to the naked eye. However, there are additional structures in the hair follicle. Hair follicles consist of an in-folding of the surface of the skin that creates an opening, sometimes called a pore. This opening extends through several layers of the skin. It is lined with sheath cells and is typically associated with a sebaceous gland. The sebaceous gland secretes sebum, which serves to condition both the hair shaft and the surface of the skin. At the bottom of the follicle is the follicular bulb, which includes the follicular papilla, and these cells make up the portion of the follicle that are principally responsible for the generation of the hair shaft.
Hair follicles have long been known to undergo periodic cycles of activity. The first phase of this “hair cycle” is known as anagen, and is divided into six sub-phases. The first five sub-phases of anagen are collectively referred to as “proanagen,” and the sixth sub-phase is referred to as “metanagen.” Commencement of metanagen is characterized by the emergence of the hair shaft above the skin surface. Anagen is followed by successive stages in which the cells of the follicle becomes apoptotic, hair growth stops, and then the cycle is resumed and the hair-producing cells of the follicle again produce a hair shaft.
In addition to combing, trimming and coloring hair, humans often seek to remove hair over certain portions of their body. While the degree to which humans desire to remove some or all of their body hair varies significantly with age, gender and culture, hair removal is widely practiced.
Hair removal is sometimes classified as depilation (the removal of the hair shaft that protrudes from the surface of the skin) or epilation (removal of the entire hair shaft, including the portion below the skin) Depilation includes hair removal by shaving and also removal by use of chemical agents such as thioglycolate. Epilation can include mechanical removal of the hair shaft (e.g. plucking, waxing, sugaring), as well as laser treatment and electrolysis.
Most methods of hair removal have only a temporary effect. The hair grows back, sometimes within a day or two. Only a few methods of hair removal are permanent (i.e. inhibit the re-growth of the hair). For example, electrolysis treatments can permanently stop hair growth.
Permanent hair removal can be desirable in many circumstances. When persons are confident that they will not change their mind about having hair present on a particular portion of their body, permanent hair removal can be an attractive alternative to daily shaving or other hair removal techniques. Unfortunately, there are several serious drawbacks associated with all known methods of permanent hair removal.
Electrolysis is performed one hair at a time, making it an impractical technique for removal of hair from anything other than very small regions of the body. Electrolysis is also painful, sometimes too painful for the person to bear.
Infrared lasers can permanently remove hair, and can be applied to areas of skin, not just individual hairs. Unfortunately, infrared laser hair removal typically works by the absorption of the infrared laser energy by pigments in the hair shaft and associated follicular cells, which causes heating that is intended to kill the hair-generating cells. This cannot be accomplished in the case of lightly colored (e.g. blonde or red) or unpigmented (e.g. gray) hair. Pigmented skin tends to absorb the infrared laser energy in the same way that pigmented follicle cells do, which interferes with laser hair removal in persons with darkly or sometimes even moderately pigmented skin.
Infrared laser hair removal is also painful and typically requires repeated treatments in order to be effective.
There remains a need for an effective means of permanent hair removal that can be used efficiently, without excessive discomfort, and on all types and colors of hair.
Photodynamic therapy (PDT) is an established therapeutic method for certain disorders. PDT is characterized by the use of (1) a phototherapeutic agent and (2) light. The phototherapeutic agent is applied or provided to the tissue or organ of interest. The light is used to cause a photo-reaction (such as photoexcitation) in either the phototherapeutic agent, or in a metabolite of the phototherapeutic agent, or in a compound produced in response to the presence of the phototherapeutic agent (the activation reaction). This reaction results in a therapeutic effect.
Early phototherapeutic agents included porphyrins such as hematoporphyrin IX, hematoporphyrin derivative, or other such molecules, including Photofrin II.
The pioneering work of Kennedy & Pottier resulted in the discovery of the use of aminolevulinic acid (ALA) as a phototherapeutic agent. ALA is a precursor to a naturally occurring molecule—protoporphyrin IX. Exposing skin to light activates protoporphyrin IX in the skin. That is, the light excites or causes a reaction in the protoporphyrin IX molecule that results in the formation of reactive free radicals. Naturally occurring protoporphyrin IX can be activated by exposure to light, but occurs in quantities too small to cause any serious effect in normal tissue. By administering exogenous ALA, cells and tissues can be caused to produce greatly increased amounts of protoporphyrin IX. The resulting high concentrations of protoporphyrin IX can result in the generation of fatal quantifies of free radicals in the target cells/tissue when protoporphyrin IX is activated by exposure to light.
Kennedy & Pottier found that ALA-induced production of protoporphyrin IX made it possible to use PDT in the treatment of several disorders of metabolically active tissues. This technology has been used in the successful commercial product Levulan®, produced by Dusa Pharmaceuticals, and which has been approved by the U.S. FDA for the treatment of actinic keratoses.
PDT has long been thought to have some effect on hair growth. Some workers have reported that photo-dynamic therapy increases the growth of hair. Others have reported that PDT can inhibit hair growth. These workers report that effective inhibition of hair growth requires that the hair shaft be mechanically removed to create a clear opening in the pore of the follicle (unobstructed by the hair shaft) so that the photodynamic agent can penetrate the follicle and, when activated, destroy the hair-generating cells. See, for example, U.S. Pat. No. 5,669,916 to Anderson, and generally Altshuler et. al. “Extended Theory of Selective Photothermolysis” Lasers in Surgery and Medicine, Vol. 29, pp 416-432 (2001).